The valve lift in existing valve actuating mechanism of an internal combustion engine mostly employs a fixed and constant structure or a non continuous variable structure. However, the internal combustion engine has different requirements on the valve actuating system under different working conditions and different speeds. When the internal combustion engine operates at a low speed, the flow inertia of gas is relatively small. At this time, if the valve lift is overlarge, a creation of inlet gas swirl will be hindered and the mixture of fuel and air will be unhomogeneous, thus hampering combustion. In this situation, under the impeding influence of throttle valves, a pump gas loss is caused, and the consumption of internal energy results in a reduction of working efficiency of part loads on the internal combustion engine. When the internal combustion engine operates at a high speed, the flow rate of gas is relatively large and the flow inertia is large too. In this situation, if the valve lift is oversmall, a strong swirl will be created in the inlet gas, which is also adverse to combustion. Therefore, a relatively large valve lift is required at this time so as to reduce swirl rate and bring about more inlet gas simultaneously.